The invention relates to a digital transmitting method and a digital transmitting apparatus and, more particularly, to a digital transmitting method and a digital transmitting apparatus for switching a plurality of transmitting or receiving antennas and transmitting and receiving a digital signal.
In recent years, a QAM (Quadrature Amplitude Modulation) transmitting system in which a digital process is applied to modulating and demodulating processes has been proposed.
In such a transmitting system, information data of, for example, 81 Mbps to be transmitted is transmitted from the transmission side at a rate of 13.5 Mega symbols/sec. namely, every about 74 nsec as symbol data of 64 values (64 points) constructed by binding on a 6-bit unit basis.
On the reception side, an equalizing process for changing filter characteristics of a demodulating unit on the reception side is executed so as to optimize an instantaneous data level to be discriminated from the data which reaches every about 74 nsec. The level of the received data is discriminated, thereby reconstructing the 6-bit data.
As mentioned above, in the QAM modulation and demodulation transmitting apparatus, it is indispensable to install the equalization processing function to change a frequency and phase characteristics on the reception side.
As for the equalizing process, for instance, refer to John G. Proakis, xe2x80x9cDIGITAL COMMUNICATIONSxe2x80x9d, the third edition, McGraw-Hill, Inc., pp. 620-653, 1995.
In the case of a transmitting apparatus as mentioned above, even if the apparatus is used in a state where the positions on the transmission side and reception side are fixed, an environmental situation in which a reflection wave is produced gradually changes due to the rain or the like.
To make it possible to follow such a change, a transmitting apparatus with an automatic equalizing function for re-setting the foregoing equalizing process in the more suitable direction in accordance with the characteristics or demodulating state of the reception data is necessary.
FIG. 9 shows a construction of a reception demodulating unit having an automatic equalizing function and it will be briefly explained. After a reception input is inputted to a variable filter 6-1 of the reception demodulating unit, a discriminator 6-2 discriminates on the basis of the level and the phase of the reception input that the reception input is data having a value closer to one of the signal points in the digital data on the transmission side.
An error detection corrector 6-3 compares levels and phases between output data of the variable filter 6-1 and the discrimination data, thereby detecting errors between them. The error detection corrector 6-3 generates a filter control signal to change filter characteristics of the variable filter 6-1 so as to reduce the errors.
FIG. 10A shows an arrangement of the signal points of the digital data on the transmission side. FIG. 10B shows the level and phase of received data before discrimination on the reception side, data after discrimination, and detected errors.
In the case where frequency characteristics are not flat on a transmission path such as, for example, the level in a high band is attenuated and a signal of a low frequency component pattern is transmitted such that the same data is continuously transmitted for a period of time of two symbols or longer and, after that, it is changed to another data, the amplitude of the data can be relatively correctly transmitted. In case of a data pattern having a lot of high band components in which data changes every symbol, however, the high band components are attenuated and the amplitude level of the data drops.
The error detection corrector 6-3 detects the errors in such a case and integrates the errors, thereby suppressing a variation in detected errors due to noises. After that, the corrector 6-3 controls so as to raise high band characteristics of the variable filter 6-1.
Thus, the automatic equalization processing function as mentioned above can cope with the gentle change in the state of the transmission path which is caused when the transmitting and receiving positions are moved, the QAM transmitting system can be used in applications which are not limited to transmission and reception at fixed positions.
A specific use example of a transmitting apparatus which is not limited to the transmission and reception at fixed positions will now be described hereinbelow. In this system, a transmission side apparatus of the transmitting apparatus having an automatic equalizing function is installed in a helicopter and a receiving apparatus is arranged in a broadcasting station building serving as a reception side and is away from the transmitting apparatus by tens of kilometers.
The helicopter transmits image information picked up by a TV camera as a radio wave from a transmitting antenna equipped to a body of the helicopter while flying, namely, moving.
The broadcasting station building serving as a reception side receives the radio wave by a receiving antenna, demodulates it and uses the demodulated image information for a television broadcasting.
Since the transmitting antenna of the helicopter cannot be attached to the bottom surface of the body due to size limitation, two transmitting antennas are inevitably attached to two positions on the right and left sides of the helicopter body. Since the direction of the broadcasting station building serving as a reception point changes depending on the flying direction of the helicopter, that transmitting antenna on either one of the right and left sides which faces the broadcasting station building is selected and the image information is transmitted by the selected antenna. A transmitting apparatus of the type having a function for automatically selecting and switching the antennas in accordance with the flying direction of the helicopter is also conceivable.
Such an example is shown in FIG. 7 and the operation will now be briefly described.
A transmission side (helicopter) 10 consists of an image compressing unit ENC, a transmission modulating unit 1, a path switching unit 2, a switching control unit 3, and transmitting antennas 4-1 and 4-2.
Compression image information from the image compressing unit ENC is QAM modulated by the transmission modulating unit 1 and is fed to an input I of the path switching unit 2. An output of the switching control unit 3 is fed to an input C of the path switching unit 2. An output Oa of the path switching unit 2 is fed to the transmitting antenna 4-1 and an output Ob is fed to the transmitting antenna 4-2 and those outputs are transmitted as a transmission signal radio wave.
A reception side (broadcasting station building) 11 consists of a receiving antenna 5, a reception demodulating unit 6, and an image decompressing unit DEC. An output of the receiving antenna 5 is fed to the reception demodulating unit 6.
The switching control unit 3 of the transmission side 10 recognizes the positional relation between the flying direction of the helicopter and the reception side 11 on the basis of information from detecting means (not shown), discriminates a proper one of the transmitting antennas 4-1 and 4-2 which can face the receiving antenna 5, and generates a corresponding switching control signal to select the proper transmitting antenna.
Thus, the path switching unit 2 is selectively switched to either the output Oa or Ob connected to the transmitting antenna which faces the reception side 11 in response to the switching control signal from the switching control unit 3. The input data is outputted from only a corresponding one of the transmitting antennas.
Since the output of the transmission modulating unit 1 can be outputted up to only a predetermined level, if it is equally distributed to a plurality of antennas, the distance at which the transmission is possible becomes short. To solve such a drawback, the selective switching method as mentioned above may be used.
The radio wave transmitted from the transmitting antenna 4-1 or 4-2 passes through the air, reaches the receiving antenna 5, is restored to the compression image information by the reception demodulating unit 6, is decompressed by the image decompressing unit DEC, and after that, it is used as image information for a television broadcasting or the like.
As described with reference to FIGS. 10A and 10B, according to the foregoing automatic equalizing function, a QAM constellation (arrangement of the signal points) state obtained by mapping the data levels before the reception data is discriminated is statistically monitored and the frequency characteristics and phase characteristics of the filter built in the reception demodulating unit on the reception side are gradually changed so as to reduce errors from the inherent constellation position.
That is, whether a demodulation QAM constellation state of the reception data is good or not cannot be momentarily discriminated from a small number of symbol data and an accurate statistic cannot be derived unless the constellation states collected over a certain extent of time, for instance, on the order of hundreds of usec are examined.
Explanation will now be made with respect to the foregoing case of the use in the helicopter or the like with reference to FIG. 8. First, it is assumed that the witching control signal of the output of a switching control unit 3 changed at time t0. In response, the path switching unit 2 switches the input I from the output Oa to the output Ob , thereby causing the output of the transmitting antenna 4-1 to almost instantaneously change from a rated value to 0 and, at the same time, causing the output of the transmitting antenna 4-2 to instantaneously change from 0 to the rated value.
Since the transmitting antenna which is used is instantaneously switched from 4-1 to 4-2 as mentioned above, the transmitting position suddenly changes which in turn causes a fading state on the reception side 11 to be suddenly changed. Consequently, the equalizing characteristics which have been in the state where they are optimum for transmission by the transmitting antenna 4-1 so far by the automatic equalizing function become improper.
That is, the constellation state is disordered after a time point near time t0 due to the improper equalization.
To prevent such a situation, the statistic process is again executed from the disordered constellation state and the demodulation of the transmission signal is executed in an improper state for a period of time until the proper equalizing characteristics are obtained, in this example, until time t1, so that transmitted compression image information cannot be correctly received.
Thus, the normal transmission is obstructed until time t1 as a result of the switching of the transmitting antennas which is performed at time t0.
It is an object of the invention to provide a data transmitting method and a data transmitting apparatus which can eliminate the foregoing drawbacks and can prevent a sudden change in characteristics on a transmission path in an antenna switching in a data transmitting apparatus and can keep a state where an automatic equalizing function on the reception side can follow a change in transmission path characteristics.
To accomplish the above object, according to one aspect of the invention, there is provided a data transmitting apparatus for transmitting and receiving information data which is digitally modulated, wherein the data transmitting apparatus operates so as to select an appropriate transmitting antenna from a plurality of transmitting antennas in accordance with a state change of a transmission path, to switch to the selected transmitting antenna, and to transmit the information data, and a soft switching control is executed in a manner such that in case of selecting and switching the transmitting antenna, a transmitting level is gently or continuously shifted from the transmitting antenna before the switching to the transmitting antenna after the switching for or over a predetermined time.
According to another aspect of the invention, there is provided a data transmitting apparatus for transmitting and receiving information data which is digitally modulated, wherein the data transmitting apparatus operates so as to select an appropriate receiving antenna from a plurality of receiving antennas in accordance with a state change of a transmission path, to switch to the selected receiving antenna, and to receive the information data, and a soft switching control of the receiving antenna is executed when selecting and switching the receiving antenna such that a supply of a reception signal to a reception demodulating unit is switched from the receiving antenna before the switching to the receiving antenna after the switching in a manner such that a receiving level is gently or continuously shifted for or over a predetermined time.
According to still another aspect of the invention, there is provided a data transmitting apparatus for transmitting and receiving information data which is digitally modulated, wherein the data transmitting apparatus operates so as to select an appropriate transmitting antenna from a plurality of transmitting antennas in accordance with a state change of a transmission path, to switch to the selected transmitting antenna and to transmit the information data and operates so as to select an appropriate receiving antenna from a plurality of receiving antennas in accordance with a state change of a transmission path, to switch to the selected receiving antenna, and to receive the information data, a soft switching control is executed in a manner such that when selecting and switching the transmitting antenna, a transmitting level is gently or continuously shifted from the transmitting antenna before the switching to the transmitting antenna after the switching over a predetermined time, and when selecting and switching the receiving antenna a soft switching control of the receiving antenna is executed such that a supply of a reception signal to a reception demodulating unit is switched from the receiving antenna before the switching to the receiving antenna after the switching in a manner such that a receiving level is gently or continuously shifted for or over a predetermined time.